Add tests for java jni bindings

Reviewed By: lucasr

Differential Revision: D4008411

fbshipit-source-id: c896a3925ff3f7fa368176a3d03c84eb7188ef60

ReactAndroid/src/main/java/com/facebook/csslayout/CSSNode.java

@@ -29,6 +29,11 @@ public class CSSNode implements CSSNodeAPI<CSSNode> {
     }
   }
 
+  /**
+   * Get native instance count. Useful for testing only.
+   */
+  static native int jni_CSSNodeGetInstanceCount();
+
   private CSSNode mParent;
   private List<CSSNode> mChildren;
   private MeasureFunction mMeasureFunction;